The invention relates generally to laser heterodyne systems and more particularly to such systems wherein a laser is used to amplify received light prior to detection of the frequency of such light.
As is known in the art, laser heterodyne systems have been used to determine the Doppler velocity of a target. Generally a laser transmits a beam of coherent light towards the target. The beam of light is reflected or scattered by the target and received by the heterodyne system. The frequency of such reflected light is offset from the frequency of the transmitted signal because of the Doppler velocity of the target. A portion of the transmitted light beam together with the received light is fed to a detector means whereby an electrical signal is produced indicative of the Doppler velocity of the target. In many applications, however, the power in the received light is relatively low and inadequate for accurately detecting the Doppler velocity of the target.
In a known heterodyne system the received light is passed through the transmitting laser for amplification prior to being fed to the detector means. That is, the laser is used as a transmitter of a beam of coherent light and simultaneously as a preamplifier for the detector means. The gain (or amplification factor) of the laser is a function of the frequency of the received light. Because the frequency of the received light is related to the Doppler velocity of the target, the preamplifying laser will not provide the most efficient gain for the received light over a range of possible Doppler velocities. Therefore, while the use of a laser for preamplification in the manner just described improves detection of Doppler velocity where the power in the received light is relatively low, in many other applications, as where the frequency of the received light is such that inadequate gain is provided by the amplifier, such technique has been found inadequate for accurately determining the target's Doppler velocity.